The Mars Global Surveyor spacecraft successfully executed its orbit insertion maneuver at Mars on September 12, 1997. The original mission plan called for a four month period of aerobraking in the upper Martian atmosphere to reduce the altitude and period of the initial orbit. The mapping phase of the mission was expected to commence in March, 1998 and continue for one complete Martian year (687 days).
One of the two MGS solar panels was damaged when it was deployed after the launch, and the damaged panel has prevented the aerobraking phase of the mission from proceeding as planned. Essentially, it is the atmospheric drag on the two panels which slows the spacecraft when it dips into the Martian atmosphere on each orbit and which therefore reduces the altitude and period of the orbit. In order to prevent permanent damage to the panel (and potential loss of the mission), aerobraking is now being conducted at a reduced pace.
The mission plan (which is of course subject to the whims of the damaged solar panel) now calls for aerobraking to proceed at the current level until May, 1998 when the maneuver must be halted as Mars approaches the Sun as viewed from Earth. This is called solar conjunction, and aerobraking will be terminated at this time because it is not possible to maintain radio contact with the spacecraft when Mars is near the Sun in the sky.
The spacecraft will then remain in a so-called phasing orbit through September, 1998. Aerobraking will be resumed in October of 1998, and the spacecraft will ultimately achieve a polar mapping orbit as originally planned in February, 1999. The mapping orbit will be nearly circular, with a period of two hours and an altitude of approximately 250 miles. During mapping, the Radio Science Team will be using a radio occultation technique to obtain profiles of the Martian atmosphere as the spacecraft enters and exits occultation by Mars on each orbit of the spacecraft. This will enable them to observe Martian weather patterns over the duration of the MGS mapping phase.
The word occult means hidden, and MGS occultations occur when the spacecraft is hidden behind Mars. Just as the spacecraft enters and exits occultation by Mars, its radio signal passes through the atmosphere of Mars on the way to Earth. The MGS Radio Science Team analyzes the changes in the characteristics of the signal at these times to determine the structure of the Martian atmosphere!
During the current aerobraking phase of the mission, the spacecraft will be occulted by Mars for a short period of time during all of the approximately 300 orbits between the end of January and the solar conjunction in May. The Radio Science Team has been given preliminary clearance to conduct their atmospheric experiments during this period. For reasons relating to the spacecraft orbit geometry, the experiments may be restricted to the entry occultations, however. If the Team conducts these experiments and produces profiles of the Martian atmosphere, then this atmospheric data will be made available to participants in the outreach program.
As of today, the spacecraft orbit period is 29.8 hours. The spacecraft orbit period at the time of the first occultation will be approximately 20 hours. By solar conjuction, the period will have been reduced to the neighborhood of 6 to 8 hours by aerobraking. The orbit geometry will allow the atmospheric measurements during the entry occultations to be made at Martian latitudes from 40N to 60S. The local times of the Martian atmospheric measurements will vary from 6 am at the end of January, to midnight by the first week of March, to 6 pm at the time of solar conjunction in May. The summer solstice in the Southern hemisphere of Mars will occur during the first week of February, and most of the atmospheric data will be acquired during the Southern summer/Northern winter. This is a very interesting time of year on Mars, and huge Martian dust storms occur with regularity.
This report will be updated as more details become available about our first set of atmospheric data from Mars, and an estimate is available about when the data will be processed and ready to share with outreach program participants.